Ground-based navigation aids are being supplanted by satellite-based aids like Global Positioning System (GPS), which make it possible for pilots to know their position with great precision anywhere in the world. With the arrival of Wide Area Augmentation System (WAAS), Satellite navigation has become accurate enough for vertical (altitude) as well as horizontal use, and is being used increasingly for instrument approaches as well as en-route navigation. However, because the GPS constellation is a single point of failure, on-board Inertial Navigation System (INS) or ground-based navigation aids are still required for backup.